The focus here is on the carbohydrates formed by dehydration synthesis. Here is the anabolic synthesis of a polymer, involving removal of a molecule of water. The reaction of course, is dehydration synthesis. Hydrolysis is a catabolic reaction involving the breakdown of a polymer by water addition across the linkages in the polymer. Here we have a straight chain drawing of a molecule of glucose. Can you identify the optically active carbons, the chiral carbons in this molecule? In fact the straight chain glucose is unstable in solution, and quickly forms a ring structure called the hemiacetal structure shown on the right. At equilibrium, 90% of the dissolved glucose is in this form. Here is that number 5 and number 1 carbon that was involved in forming that hemiacetal in the first place. A consequence of hemiacetal formation is that the hydroxide on the number 1 carbon can end up either above or below the plane of the ring, creating a ‘racemic’ mixture of ‘enantiomers’ of glucose called alpha- and beta- glucose. The alpha- and beta- glucose are present in equal proportions in a racemic mixture of hemiacetal glucose. In this slide we’re going to see the formation of a glucose dimer by water removal (a condensation reaction). Here, an alpha 1-4 glycosidic linkage has formed between the 2 sugars, linking the number 1 carbon of glucose on the left, with the number 4 carbon of glucose on the right. Repetitive addition of alpha-glucose monomers to a growing chain can form starches in plants and glycogen in animals. The glycoside linkages, which could include alpha-1-4, like the one shown here, as well as alpha-1 6 and other alpha-linkages, are characteristic of flexible storage polysaccharides that eventually are hydrolyzed to get the sugar out to be metabolized for energy. If beta-glucose enantiomers are polymerized in condensation reactions, like the one shown here, the beta-glycoside linkages that form (here it’s a beta 1-4 glycoside linkage) create a rigid inflexible polysaccharide like cellulose. Criss-crossing laminas of cellulose are laid down to form plant cell walls. Not shown here, a modified sugar called N-acetyl glucosamine, will polymerize to form chitin, the main component of fungal cell walls and the major component of the hard exoskeleton of insects and crustaceans like lobsters. Take a quick look at the difference in structure between storage and structural polysaccharides. Starch is actually made up of unbranched alpha 1-4 glycoside-linked glucose polysaccharides called amylose, and a branched glucose polysaccharide called amylopectin. Glycogen in animal cells is just a more highly branched polysaccharide of alpha-linked glucose molecules. Because the beta glycoside linkages are inflexible, cellulose is pretty much a linear polymer that can form hydrogen bonds along their length, with other cellulose polymers to form the sheet-like lamina structure illustrated here. It’s these tough unfolded structures that give plant cell walls their strength and allow trees to grow to gravity-defying heights. You should recall that eukaryotic cells are sugar coated, as illustrated in this drawing of the fluid mosaic membrane. This means that short polysaccharides, or oligosaccharides, are covalently bound to proteins, or to the heads of phospholipids facing outside the cell. The combinations of oligosaccharides and either membrane proteins or membrane phospholipids, are called glycoproteins or glycolipids respectively. Note that the sugars are not associated with the cytoplasmic surface of the plasma membrane.